Electroplating bath for depositing a black chromium layer and method for electroplating a black chromium layer on a substrate

ABSTRACT

The present invention relates to a very specific electroplating bath for depositing a black chromium layer, and a respective method for electroplating such a layer. The electroplating bath comprises two specific groups of compounds defined as (D) and (E), present in a particularly defined molar ratio ranging from 0.9 to 2.65, based on (E):(D). The black chromium layer is excellently suited for decorative purposes.

FIELD OF THE INVENTION

The present invention relates to a very specific electroplating bath fordepositing a black chromium layer, and a respective method forelectroplating such a layer. The electroplating bath comprises twospecific groups of compounds defined as (D) and (E), present in aparticularly defined molar ratio ranging from 0.9 to 2.65, based on(E):(D). The black chromium layer is excellently suited for decorativepurposes.

BACKGROUND OF THE INVENTION

From the very beginning of chromium coatings, a high interest in blackchromium coatings was observable due to its great appeal for opticalapplications.

Beginning with even black hexavalent chromium coatings, the focus todaysignificantly shifted to trivalent chromium coatings due to a higherenvironmental acceptance. Since some years the demand for dark, evenneutral deep dark (also named neutral black), trivalent chromiumcoatings, is more and more increasing, for example for decorativeautomotive parts. However, since such a neutral black color tone mightbe perceived as too cold in some cases, a slight color tone modificationis often demanded, which does not compromise the deep black tone itselfbut is adding a tiny warmth to it to create a more warm black colortone. In principle, both the neutral black color tone as well as thewarm black color tone have strong demand in industry although being verysimilar.

However, the degree of black varies significantly and depends ondeposition parameters as well as bath ingredients.

In many cases, the black color obtained from trivalent chromium coatingswas not black enough to meet either a neutral black or a warm blackcolor tone, for example to meet requirements for decorative parts in theautomotive industry. In other cases, darkness was met but the overalloptical impression is not sufficient. In even other cases aninsufficient color stability over time was obtained.

WO 2012/150198 A2 refers to a method and plating bath forelectrodepositing a dark chromium layer.

WO 2017/053655 A1 refers to a method for adjusting the lightness L* bymeans of an activated carbon filter as well as a dark electroplatedtrivalent chromium layer on a workpiece.

CN 107099824 B refers to a black chromium electroplating solution, acomposite plating layer and a preparation method thereof. The trivalentblack chromium coating formed by this black chromium electroplatingsolution has a deep black and strong uniform coverage.

US 2020/094526 A1 refers to a black plated resin part comprising a blackchromium plating layer exhibiting an b* value of 3.0 or less.

According to US'526 significant progress was made regarding a neutralblack color tone and a warm black color tone. However, in all theseattempts further improvements are needed for an industrial utilization.For example, in many cases the desired color tone is only obtainedthrough unacceptably long idle times to initiate a natural color ageing,which finally results in the desired color tone. In other cases, thedesired color tone is obtained rather quickly but the deposition is notpossible on geometrically sophisticated substrates due to hazeformation, burnings and skip plating.

Thus, there is a great demand to further improve the available methodsand plating baths in order to overcome said problems.

Objective of the Invention

It was therefore the objective of the present invention to provide anelectroplating bath and a respective method for electroplating, whichallows on the one hand a quick and stable color tone formation (for botha neutral black color tone as well as warm black color tone) and on theother hand allows deposition on a broad variety of substrate geometrieswithout plating defects and thus, providing an excellent opticalappearance. Furthermore, both color tones should be specifically andeasily targeted and thus obtained.

DETAILED DESCRIPTION OF THE INVENTION

This objective is solved by means of the present invention, anelectroplating bath and a respective method for electroplating.

Therefore, the present invention refers to an electroplating bath fordepositing a black chromium layer, the electroplating bath comprising:

-   -   (A) trivalent chromium ions;    -   (B) one or more than one complexing agent for said trivalent        chromium ions;    -   (C) optionally, one or more than one pH buffer compound for said        electroplating bath;    -   (D) one or more than one compound comprising at least one —SON        moiety, salts, esters, and/or isoforms thereof; and    -   (E) one or more than one organic compound, including sulfoxides        thereof, comprising at least one —SH moiety and/or at least one        —S—(CH₂)_(k)—CH₃ moiety, wherein k is an integer ranging from 0        to 4,    -   characterized in that    -   (E) and (D) are present in a molar ratio ranging from 0.9 to        2.65, based on (E):(D).

Own experiments have shown (see examples below) that above mentionedproblems strongly relate to the molar ratio of (E) to (D) and can besolved by maintaining the molar ratio in a range as defined above. Thedesired color tones (either neutral black or warm black) are quicklyformed. Furthermore, they can be formed even on substrates withsophisticated geometries such that a very uniform deposition quality isprovided without plating defects. However, it furthermore turned outthat these excellent results are obtained only if the narrowly definedmolar ratio range as defined above is kept.

Very preferably, the black chromium layer is a decorative chromiumlayer. Typical applications are automotive parts, most preferably forthe interior of a car. The electroplating bath of the present inventionis very suitable in order to obtain such a black chromium layer, mostpreferably such a black chromium layer as defined throughout the presenttext.

The black chromium layer in the context of the present invention is verymuch preferably defined by the L*a*b* color system, preferably asintroduced in 1976 by the Commission Internationale de l'Eclairage, ifnot stated otherwise.

Generally preferred is an electroplating bath of the present invention,wherein the black chromium layer has an L* value of 50 or below,preferably of 49 or below, more preferably of 48 or below, even morepreferably 47 or below, yet even more preferably 46 or below, furthermore preferably 45 or below, most preferably 43 or below. An L* value of50 or below is typically well perceived as black and dark. Generally,the lower the L* value (preferably as defined above) the stronger theimpression of a black/dark color tone.

Preferred is an electroplating bath of the present invention, whereinthe black chromium layer has an a* value ranging from −1.5 to +3,preferably ranging from −1 to +2.5, most preferably ranging from −0.5 to+2. Preferably, the a* value is at least positive. Most preferably thisapplies to the neutral black color tone and to the warm black colortone.

A distinction between a neutral black color tone and a warm black colortone is typically based on slightly different b* values.

In some cases, preferred is an electroplating bath of the presentinvention, wherein the black chromium layer is a neutral black chromiumlayer. This more preferably means that an electroplating bath of thepresent invention is preferred, wherein the black chromium layer has anb* value ranging from −2.5 to +2.9, preferably ranging from −2 to +2,more preferably ranging from −1.5 to +1.5, most preferably ranging from−1 to +1.

For a neutral black color tone, most preferably the L* value is 45 orbelow, more preferably 44 or below, even more preferably 43 or below,yet even more preferably 42 or below, most preferably 41 or below.

In other cases, preferred is an electroplating bath of the presentinvention, wherein the black chromium layer is a chromium layer having awarm black color tone. This more preferably means that an electroplatingbath of the present invention is preferred, wherein the black chromiumlayer has an b* value ranging from +3 to +6, preferably ranging from+3.5 to +5.8, most preferably from +4 to +5.5.

In the context of the present invention, both the neutral black colortone as well as the warm black color tone can be obtained, which is ofgreat benefit.

Compound (A) and General Bath Compounds:

The electroplating bath of the present invention is preferably aqueous,i.e. it comprises water, preferably at least 55 vol.-% or more is water,based on the total volume of the electroplating bath, more preferably 65vol.-% or more, even more preferably 75 vol.-% or more, yet even morepreferably 85 vol.-% or more, still more preferably 90 vol.-% or more,most preferably 95 vol.-% or more. Most preferably, water is the onlysolvent.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is acidic, preferably having a pH ranging from1.5 to 5.0, more preferably from 2.0 to 4.6, even more preferably from2.4 to 4.2, yet more preferably from 2.7 to 3.8, most preferably from3.0 to 3.5. The pH is preferably adjusted with hydrochloric acid,sulfuric acid, ammonia, potassium hydroxide, and/or sodium hydroxide.

The electroplating bath of the present invention comprises (A) trivalentchromium ions.

Preferred is an electroplating bath of the present invention, whereinthe trivalent chromium ions have a total concentration ranging from 5g/L to 35 g/L, based on the total volume of the electroplating bath,preferably from 6 g/L to 32 g/L, more preferably from 7 g/L to 29 g/L,even more preferably from 8 g/L to 26 g/L, yet even more preferably from9 g/L to 23 g/L, most preferably from 10 g/L to 22 g/L.

Preferably, the trivalent chromium ions are from a trivalent chromiumsalt, preferably from an inorganic chromium salt and/or an organicchromium salt, most preferably from an inorganic chromium salt. Apreferred inorganic chromium salt comprises chloride and/or sulfateanions, preferably sulfate anions. A very preferred inorganic chromiumsalt is basic chromium sulfate. A preferred organic chromium saltcomprises carboxylic acid anions, preferably formate, acetate, malate,and/or oxalate anions.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath comprises sulfate ions, most preferably from atrivalent chromium salt. Sulfate ions are excellently contribute to theconductivity of the electroplating bath.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, a compound comprising chromium with an oxidation number +6.Thus, the electroplating bath is substantially free of, preferably doesnot comprise, hexavalent chromium.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, cobalt ions. Preferably, the black chromium layer issubstantially free of, preferably does not comprise, cobalt. Only invery rare cases it is also preferred that the electroplating bath andthe black chromium layer, respectively, comprise cobalt, although thisis less preferred. However, if cobalt is present, preferably in theblack chromium layer more chromium is present than cobalt. The latterpreferably means that the atom ratio of chromium to cobalt (i.e. Cr:Co)is more than 1, preferably 2 or more, more preferably 3 or more, mostpreferably 4 or more. This is most preferably based on the total amountof chromium and cobalt atoms in the black chromium layer.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, nickel ions. In some cases, a typical Ni-contamination of upto 150 ppm is observed, which is basically acceptable and thereforeconsidered as substantially free of nickel ions. Thus, in some cases theelectroplating bath of the present invention preferably comprises nickelions in a concentration ranging from 0 ppm to 200 ppm, based on thetotal weight of the electroplating bath, preferably from 1 ppm to 150ppm, most preferably from 2 ppm to 100 ppm. Preferably, the blackchromium layer is substantially free of, preferably does not comprise,nickel.

It is generally preferred to avoid such environmental questionablenickel and cobalt ions. This generally leads to less complicatedwastewater treatment and bath disposal. In addition, neither nickel norcobalt is needed to obtain the neutral black color tone and the warmblack color tone.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, fluoride ions. Preferably, the black chromium layer issubstantially free of, preferably does not comprise, fluorine.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, compounds containing fluorine. This most preferably comprisesfluorine-containing surface-active compounds. They are in particular notdesired due to increased environmental limitations.

Preferred is an electroplating bath of the present invention, whereinthe electroplating bath is substantially free of, preferably does notcomprise, phosphate anions, more preferably is substantially free of,preferably does not comprise, phosphorous-containing compounds.Preferably, the black chromium layer is substantially free of,preferably does not comprise, phosphorous. However, this does notexclude phosphorous in a subsequent layer deposited onto the blackchromium layer, e.g. a passivation layer.

Preferred is an electroplating bath of the present invention furthercomprising halogen anions, preferably chloride anions. In the context ofthe present invention this is preferred and respective electroplatingbaths are named chloride-containing baths. More preferred is anelectroplating bath of the present invention comprising chloride ionsand sulfate ions.

Preferred is an electroplating bath of the present invention, whereinthe chloride ions have a concentration ranging from 50 g/L to 200 g/L,based on the total volume of the electroplating bath, preferably rangingfrom 60 g/L to 185 g/L, more preferably ranging from 70 g/L to 170 g/L,even more preferably ranging from 80 g/L to 155 g/L, most preferablyranging from 90 g/L to 140 g/L. Chloride ions are preferably from achloride salt and/or hydrochloric acid, preferably from sodium chloride,potassium chloride, ammonium chloride, chromium chloride (at least as apart of all chloride ions), and/or mixtures thereof. Typically, chlorideions are present as the anion of a conductivity salt as preferablymentioned before. A very preferred conductivity salt is ammoniumchloride, sodium chloride and potassium chloride, ammonium chloridebeing preferred most.

Preferred is an electroplating bath of the present invention furthercomprising bromide anions. This typically avoids an anodic formation ofundesired hexavalent chromium species. Preferably, the bromide ions havea concentration ranging from 3 g/L to 20 g/L, based on the total volumeof the electroplating bath, preferably ranging from 4 g/L to 18 g/L,more preferably ranging from 5 g/L to 16 g/L, even more preferablyranging from 6 g/L to 14 g/L, most preferably ranging from 7 g/L to 12g/L. Bromide ions are preferably from a bromide salt, preferably fromsodium bromide, potassium bromide, ammonium bromide, and/or mixturesthereof.

More preferred is an electroplating bath of the present inventioncomprising chloride ions, bromide ions, and sulfate ions, most preferredwith concentrations as defined throughout the present text as beingpreferred.

In some cases an electroplating bath of the present invention ispreferred further comprising Fe(II) ions, preferably in a concentrationranging from 0.1 mmol/L to 10 mmol/L, based on the total volume of theelectroplating bath, preferably from 0.4 mmol/L to 8 mmol/L, morepreferably from 0.6 mmol/L to 6 mmol/L, even more preferably from 0.8mmol/L to 5 mmol/L, most preferably from 1 mmol/L to 4 mmol/L. This isin particular preferred if the electroplating bath of the presentinvention comprises chloride ions. Thus, most preferred is anelectroplating bath of the present invention comprising chloride ions,bromide ions, sulfate ions, and Fe(II) ions, most preferred withconcentrations as defined throughout the present text as being preferredfor them. Fe(II) ions are preferably from a respective iron salt,preferably from an iron (II) sulfate salt. Typically, iron ions haveseveral beneficial effects on the electroplating performance and on thedeposited black chromium layer obtained by the present invention. Inmany cases, an increased electroplating rate is observed which allows athicker layer thickness. Preferably, the black chromium layer comprisesiron, preferably up to 15 at.-%, based on all atoms in the blackchromium layer, more preferably up to 12 at.-%, even more preferred upto 10 at.-%, yet even more preferably up 8 at.-%, most preferably up to6 at.-%.

Furthermore very preferred is an electroplating bath of the presentinvention, wherein the trivalent chromium ions and the Fe(II) ions (ifpresent) are the only transition metals in the plating bath, mostpreferably chromium ions and iron ions (if present) are the onlytransition metals in the plating bath. An exception are Nicontaminations as mentioned already above, which are generallyacceptable and therefore preferably included.

In some cases, an electroplating bath of the present invention ispreferred further comprising at least one sulfur-containing compoundbeing different from (D) and (E).

In some cases, an electroplating bath of the present invention ispreferred further (i.e. in addition to (D) and (E)) comprising saccharinand/or salts thereof.

In some cases, an electroplating bath of the present invention ispreferred further (i.e. in addition to (D) and (E)) comprising asulfur-containing diol, most preferably in addition to above mentionedsaccharin and/or salts thereof.

Preferred is an electroplating bath of the present invention furthercomprising at least one surface-active compound. A preferredsurface-active compound comprises a cationic or an anionicsurface-active compound, preferably an anionic surface-active compound.A preferred anionic surface-active compound comprises sulfosuccinates,alkyl benzene sulfonates having 8 to 20 aliphatic carbon atoms, alkylsulfates having 8 to 20 carbon atoms, and/or alkyl ether sulfates.Preferably, the at least one surface-active compound is free of fluorineatoms. Most preferably, the at least one surface-active compound is nota compound of (D) and (E). In other words, preferably (D) and (E) arenot surface-active compounds.

Preferred sulfosuccinates comprise sodium diamyl sulphosuccinate.

Preferred alkyl benzene sulfonates having 8 to 20 aliphatic carbon atomscomprise sodium dodecyl benzene sulfonate.

Preferred alkyl sulfates having 8 to 20 carbon atoms comprise sodiumlauryl sulfate.

Preferred alkyl ether sulfates fatty alcohols comprise sodium laurylpolyethoxy sulfates.

Preferred is an electroplating bath of the present invention, whereinthe at least one surface-active compound has a total concentrationranging from 0.001 g/L to 0.05 g/L, based on the total volume of theelectroplating bath, preferably from 0.005 g/L to 0.01 g/L.

In contrast, in some cases an electroplating bath of the presentinvention is preferred, wherein the electroplating bath is substantiallyfree of, preferably does not comprise, chloride ions, preferably doesnot comprise halogen anions. In the context of the present inventionthis is less preferred and respective electroplating baths are namedchloride-free baths. In such a case, the electroplating bath of thepresent invention preferably comprises sulfate ions to compensate themissing chloride ions. Even more preferred, the electroplating bath ofthe present invention comprises sulfate ions in addition to the sulfateions from the chromium salt, most preferably by means of a conductivesalt. A very preferred conductive salt is potassium sulfate, sodiumsulfate, ammonium sulfate, or mixtures thereof. In this particular case,the electroplating bath is preferably in some cases substantially freeof, preferably does not comprise, bromide ions. However, it is preferredin some rare cases that such an electroplating bath comprises iron ions,preferably Fe(II) ions, most preferably in the concentrations as definedabove.

Compound (B):

The electroplating bath of the present invention comprises (B) one ormore than one complexing agent for said trivalent chromium ions. Suchcompounds keep the trivalent chromium ions in solution. Preferably, theone or more than one complexing agent is not a compound of (D) and (E)and is therefore preferably different from (D) and (E).

Preferred is an electroplating bath of the present invention, whereinthe one or more than one complexing agent comprises an organic acidand/or salts thereof, preferably an organic carboxylic acid and/or saltsthereof, most preferably an organic carboxylic acid comprising one, two,or three carboxylic groups and/or salts thereof.

The organic carboxylic acid and/or salts thereof (preferably also theorganic carboxylic acid comprising one, two, or three carboxylic groupsand/or salts thereof) are preferably substituted with a substituent orunsubstituted. A preferred substituent comprises an amino group and/or ahydroxyl group. Preferably, the substituent does not comprise a SHmoiety and/or a SCN moiety.

More preferably, the organic carboxylic acid and/or salts thereof(preferably also the organic carboxylic acid comprising one, two, orthree carboxylic groups and/or salts thereof) comprise amino carboxylicacids (preferably alpha-amino carboxylic acids), hydroxyl carboxylicacids, and/or salts thereof. Preferred (alpha-) amino carboxylic acidscomprise glycine, aspartic acid, and/or salts thereof. Preferably, theamino carboxylic acids (preferably alpha-amino carboxylic acids,respectively) is not a compound according to (E), more preferably is nota sulfur-containing amino carboxylic acid (preferably is not asulfur-containing alpha-amino carboxylic acid, respectively), mostpreferably is not methionine. It is in particularly preferred that theone or more than one complexing agent is distinct from the compound offormula (E).

More preferred is an electroplating bath of the present invention,wherein the one or more than one complexing agent comprises formic acid,acetic acid, oxalic acid, tartaric acid, malic acid, citric acid,glycine, aspartic acid, and/or salts thereof, preferably formic acid,acetic acid, oxalic acid, tartaric acid, malic acid, citric acid, and/orsalts thereof, more preferably formic acid, acetic acid, oxalic acid,tartaric acid, malic acid, and/or salts thereof, even more preferablyformic acid, acetic acid, and/or salts thereof, most preferably formicacid and/or salts thereof. This most preferably applies, if theelectroplating bath of the present invention comprises chloride ions. Incontrast, if the electroplating bath of the present invention ischloride-free, preferably the one or more than one complexing agentcomprises oxalic acid, tartaric acid, malic acid, citric acid, and/orsalts thereof, most preferably malic acid and/or salts thereof.

Preferred is an electroplating bath of the present invention, whereinthe one or more than one complexing agent has a total concentrationranging from 5 g/L to 200 g/L, based on the total volume of theelectroplating bath, preferably ranging from 8 g/L to 150 g/L, morepreferably ranging from 10 g/L to 100 g/L, even more preferably from 12g/L to 75 g/L, yet even more preferably ranging from 15 g/L to 50 g/L,most preferably ranging from 20 g/L to 35 g/L. This most preferablyapplies if the electroplating bath comprises chloride ions but generallyalso applies to a chloride-free electroplating bath.

If the electroplating bath of the present invention is particularlychloride-free, the one or more than one complexing agent has a totalconcentration ranging from 5 g/L to 100 g/L, based on the total volumeof the electroplating bath, preferably ranging from 5.5 g/L to 75 g/L,more preferably ranging from 6 g/L to 50 g/L, even more preferably from6.5 g/L to 25 g/L, yet even more preferably ranging from 7 g/L to 18g/L, most preferably ranging from 7.5 g/L to 13 g/L. This preferablyapplies to oxalic acid, tartaric acid, malic acid, citric acid, andsalts thereof, most preferably to malic acid and salts thereof.

Preferred is an electroplating bath of the present invention, wherein(B)/(A) forms a molar ratio ranging from 1 to 1.5, preferably rangingfrom 1.1 to 1.4, most preferably ranging from 1.2 to 1.3.

Compound (C):

The electroplating bath of the present invention comprises (C)optionally, one or more than one pH buffer compound for saidelectroplating bath. Most preferably, the electroplating bath of thepresent invention comprises (i.e. not optionally) one or more than onepH buffer compound. In the latter case, an electroplating bath of thepresent invention is preferred, wherein the one or more than one pHbuffer compound for said electroplating bath is distinct (i.e.different) from (B). In such a case, the one or more than one pH buffercompound does not comprise a carboxylic acid, preferably does notcomprise an organic acid. In such a case they are counted to (B).

In many cases an electroplating bath of the present invention ispreferred, wherein the one or more than one pH buffer compound comprisesa boron-containing compound, preferably boric acid and/or a borate, mostpreferably boric acid. A preferred borate is sodium borate.

Very preferred is an electroplating bath of the present invention,wherein the one or more than one pH buffer compound has a totalconcentration ranging from 30 g/L to 250 g/L, based on the total volumeof the electroplating bath, preferably ranging from 35 g/L to 200 g/L,more preferably ranging from 40 g/L to 150 g/L, even more preferablyranging from 45 g/L to 100 g/L, most preferably ranging from 50 g/L to75 g/L. This even more preferably applies to said boron-containingcompound, yet even more preferably to said boric acid together with saidborate, most preferably to said boric acid. Most preferably the one ormore than one pH buffer compound comprises boric acid but no borate.Thus, most preferred is an electroplating bath of the present invention,wherein (C) comprises boric acid, preferably in a total amount rangingfrom 35 g/L to 90 g/L, based on the total volume of the electroplatingbath, preferably from g/L to 80 g/L, more preferably from 50 g/L to 70g/L, most preferably from 56 g/L to 66 g/L.

In some other cases the electroplating bath of the present inventiondoes not explicitly comprise a distinct pH buffer compound. Rather, theone or more than one complexing agent for said trivalent chromium ionsare present in such an amount and selected in such a way that they donot only serve as complexing agent for the trivalent chromium ions butadditionally serve as pH buffer compound. In the context of the presentinvention this is less preferred but possible.

Compound (D):

The electroplating bath of the present invention comprises (D) one ormore than one compound comprising at least one —SCN moiety, salts,esters, and/or isoforms thereof. The term “—SCN moiety” denotes athiocyanate moiety or group, respectively.

Preferably, said compound is organic and/or inorganic, preferablyinorganic. Preferred organic compounds comprise an alkyl and/or arylcompound thereof, preferably substituted or unsubstituted.

Preferred is an electroplating bath of the present invention, wherein in(D) said compound has all together 1 to 30 carbon atoms, preferably 2 to25, more preferably 3 to 20, even more preferably 4 to 17, mostpreferably 5 to 14. This most preferably applies if said compound is anorganic compound.

Preferably, in (D) said compound has all together 1 to 10 carbon atoms,more preferably 1 to 8, even more preferably 1 to 6, most preferably 1to 4.

Very preferably in (D) said compound has one single carbon atom only,most preferably (D) comprises at least thiocyanic acid, isoforms, and/orsalts thereof, preferably at least thiocyanic acid and/or salts thereof.Preferably the salt comprises potassium thiocyanate and/or sodiumthiocyanate.

The term “acid” in “thiocyanic acid” includes itsdeprotonated/dissociated form.

A preferred isoform thereof is isothiocyanic acid and/or salts thereof.

In the context of the present invention, (D) is present in a totalamount ranging from 100 mmol/L to 750 mmol/L, based on the total volumeof the electroplating bath, preferably in a total amount ranging from100 mmol/L to 600 mmol/L, preferably from 100 mmol/L to 450 mmol/L, morepreferably from 100 mmol/L to 300 mmol/L, even more preferably from 115mmol/L to 250 mmol/L, most preferably from 130 mmol/L to 200 mmol/L.

However, preferred is an electroplating bath according to the presentdisclosure, wherein said bath comprises (D) in a total amount rangingfrom 20 mmol/L to 750 mmol/L, based on the total volume of theelectroplating bath, preferably from 50 mmol/L to 600 mmol/L, morepreferably from 75 mmol/L to 450 mmol/L, even more preferably from 100mmol/L to 300 mmol/L, yet even more preferably from 115 mmol/L to 250mmol/L, most preferably from 130 mmol/L to 200 mmol/L. Thus, in thiscase (D) has not a lower total concentration limit of 100 mmol/L butrather the ranges as defined above. However, all other features asdefined throughout the present text for the electroplating bathpreferably still apply also to this specific disclosure.

The aforementioned concentration ranges most preferably apply to saidcompound in (D) having preferably all together 1 to 10 carbon atoms,more preferably 1 to 8, even more preferably 1 to 6, most preferably 1to 4. Even most preferably it applies to thiocyanic acid, isoforms,and/or salts thereof.

Preferably, concentration ranges above are based on SCN⁻, i.e. are basedon monobasic thiocyanate and thiocyanate moieties, respectively.

Furthermore, it is explicitly preferred that above concentrations can befreely combined to form not explicitly disclosed concentration ranges.This includes most preferably further combinations of lower limits withhigher limits not explicitly mentioned throughout the text.

Compound (E):

The electroplating bath of the present invention comprises (E) one ormore than one organic compound, including sulfoxides thereof, comprisingat least one —SH moiety and/or at least one —S—(CH₂)_(k)—CH₃ moiety,wherein k is an integer ranging from 0 to 4.

Preferably, k is 0, 1, 2, 3, or 4, preferably 0, 1, or 2. The term “—SHmoiety” denotes a thiol or sulfhydryl moiety or group, respectively.

In the context of the present invention “sulfoxides” thereof denotes anoxygen chemically linked via a double bond to a sulphur atom, i.e. saidorganic compound also comprises —S(═O)—(CH₂)_(k)—CHs moieties.

Preferred is an electroplating bath of the present invention, whereinsaid bath comprises (E) in a total amount ranging from 1 mmol/L to 950mmol/L, based on the total volume of the electroplating bath, preferablyfrom 50 mmol/L to 800 mmol/L, more preferably from 100 mmol/L to 650mmol/L, even more preferably from 140 mmol/L to 550 mmol/L, yet evenmore preferably from 180 mmol/L to 500 mmol/L, most preferably from 195mmol/L to 450 mmol/L.

Preferred is an electroplating bath of the present invention, wherein in(E) said organic compound independently comprises an amino moiety.

Preferred is an electroplating bath of the present invention, wherein in(E) said organic compound independently comprises a carboxylic acidmoiety and/or salts thereof.

Preferred is an electroplating bath of the present invention, wherein in(E) said organic compound independently comprises an amino acid and/orsalts thereof, preferably an alpha-amino acid.

Preferred is an electroplating bath of the present invention, wherein(E) comprises at least a compound of formula (I), salts, and/orsulfoxides thereof

R¹—S—(CH₂)_(n)—CH(NH₂)—R²  (I),

-   -   wherein        -   R¹ is a branched or unbranched C1 to C4 alkyl,        -   R² is selected from the group consisting of COOH, salts            thereof, and (CH₂)_(m)—OH,        -   n is an integer ranging from 1 to 4, and        -   m is an integer ranging from 1 to 4.

Preferred is an electroplating bath of the present invention, wherein R¹is methyl, ethyl, n-propyl, or iso-propyl, preferably methyl or ethyl,most preferably methyl.

Preferred is an electroplating bath of the present invention, wherein R²is COOH and/or salts thereof. Preferably, COOH includes also thedeprotonated/dissociated form thereof.

Preferred is an electroplating bath of the present invention, wherein nis 1 or 2, preferably 2.

Preferred is an electroplating bath of the present invention, wherein mis 1 or 2.

Preferred is an electroplating bath of the present invention, wherein(E) comprises at least methionine.

According to own experiments (see in the text below under “Examples”) ifthe molar ratio of (E):(D) is outside a range from 0.9 to 2.65 nosufficiently black chromium layer is obtained. Most important, if themolar ratio is exceeding 2.65, the heat-treating in the method of thepresent invention does not significantly affect the final colorformation.

Furthermore, if the molar ratio is below 0.9 the number of platingdefects is increasing, in particular on substrates with sophisticatedsurface geometry. Own hull cell experiments have shown that with a molarratio significantly below 0.9 an unacceptable small range of currentdensities is available. However, this is not acceptable in view oftechnical demands.

In contrast, these problems are overcome with the present invention.

Preferred is an electroplating bath of the present invention, whereinthe molar ratio of (E):(D) is ranging from 0.95 to 2.6, preferably from1 to 2.55, more preferably from 1.1 to 2.5, even more preferably from1.2 to 2.45, most preferably from 1.25 to 2.4.

In some cases preferred is an electroplating bath of the presentinvention, wherein the molar ratio of (E):(D) is ranging from 0.9 to2.5, preferably from 0.95 to 2, more preferably from 1 to 1.8, yet evenmore preferably from 1.05 to 1.5, most preferably from 1.1 to 1.3. Theseare in some cases preferred molar ranges for particularly a warm blackcolor tone.

Thus, a respective electroplating bath of the present invention ispreferably for a black chromium layer having a b* value ranging from +3to +5.5, according to the L*a*b* color system, preferably ranging from+3.5 to +5.0.

In other cases preferred is an electroplating bath of the presentinvention, wherein the molar ratio of (E):(D) is ranging from 1.6 to2.65, preferably from 1.9 to 2.6, more preferably from 2.05 to 2.55,even more preferably from 2.1 to 2.5, yet even more preferably from 2.15to 2.45, most preferably from 2.2 to 2.4. These are in some casesparticularly preferred molar ranges for a neutral black color tone.

Thus, a respective electroplating bath of the present invention ispreferably for a black chromium layer having a b* value ranging from−1.5 to +1.5, according to the L*a*b* color system, preferably rangingfrom −1 to +1.0.

Method for Electroplating:

The present invention furthermore relates to a method for electroplatinga black chromium layer on a substrate, the method comprising the steps

-   -   (a) providing the substrate,    -   (b) contacting the substrate with an electroplating bath        according to the present invention, preferably an electroplating        bath as described as being preferred throughout the present        text, or according to the present disclosure (see text above),    -   (c) applying an electrical current such that the black chromium        layer is electroplated onto the substrate,    -   (d) heat-treating the substrate obtained from step (c) at a        temperature ranging from 30° C. to 100° C.

The aforementioned features regarding the electroplating bath of thepresent invention (or according to the present disclosure), includingthe preferred variants thereof, preferably apply likewise to the methodfor electroplating of the present invention, most particular to step (b)of said method. Furthermore, the aforementioned regarding the L*a*b*values (and possibly other parameters of the black chromium layer) mostpreferably applies to the black chromium layer electroplated in step(c).

In step (a) the substrate is provided.

In some cases, a method of the present invention is preferred, whereinthe substrate comprises a plastic substrate, preferably is a plasticsubstrate. In other cases, a method of the present invention ispreferred wherein the substrate comprises a metallic substrate,preferably is a metallic substrate.

In many cases a method of the present invention is preferred, wherein instep (a) the substrate comprises a thermoplastic substrate, preferablyan amorphous thermoplastic substrate and/or a semi-crystallinethermoplastic.

More preferred is a method of the present invention, wherein in step (a)the substrate comprises butadiene moieties, preferably polybutadiene.

Also preferred is a method of the present invention, wherein in step (a)the substrate comprises nitrile moieties.

Also preferred is a method of the present invention, wherein in step (a)the substrate comprises acryl moieties.

Very preferred is a method of the present invention, wherein in step (a)the substrate comprises polymerized styrene.

Most preferred is a method of the present invention, wherein in step (a)the substrate comprises acrylonitrile butadiene styrene (ABS),acrylonitrile butadiene styrene-polycarbonate (ABS-PC), polypropylene(PP), polyamide (PA), polyetherimide (PEI), a polyetherketone (PEK), ormixtures thereof, preferably acrylonitrile butadiene styrene (ABS)and/or acrylonitrile butadiene styrene-polycarbonate (ABS-PC). Suchplastic substrates are typically used in decorative applications such asautomotive parts, in particular ABS and ABS-PC.

Preferred is a method of the present invention, wherein thepolyetherketone (PEK) comprises polyaryletherketone (PAEK), poly etherether ketone (PEEK), poly ether ether ether ketone (PEEEK), poly etherether ketone ketone (PEEKK), poly ether ketone ether ketone ketone(PEKEKK), poly ether ketone ketone (PEKK), and/or mixtures thereof,preferably poly ether ether ketone (PEEK), polyaryletherketone (PAEK),and/or mixtures thereof.

In some cases, a method of the present invention is preferred, whereinthe substrate is a metallic substrate, preferably comprising iron,copper, nickel, aluminum, zinc, mixtures thereof, and/or alloys thereof.A very preferred metallic substrate comprising iron is steel. A mixturethereof preferably includes composites.

Preferred is a method of the present invention further comprising priorto step (b) at least one metal plating step to deposit at least onemetal layer, most preferably at least one nickel plating step to depositat least one nickel layer. In many cases two or even three such metalplating steps are preferably involved.

Most preferably, the at least one nickel layer comprises at least onebright-nickel layer and/or (preferably or) at least one satin nickellayer, most preferably at least one bright-nickel layer.

More preferred is a method of the present invention, wherein the atleast one nickel layer comprises at least one semi-bright nickel layer,preferably at least one semi-bright-nickel layer in addition to said atleast one bright-nickel layer and/or said at least one satin nickellayer. The at least one semi-bright nickel layer is preferablyoptionally. Most preferably (if applied) the at least one semi-brightnickel layer is deposited prior to said at least one bright-nickel layerand/or said at least one satin nickel layer.

Also preferred is a method of the present invention, wherein the atleast one nickel layer comprises at least one MPS nickel layer,preferably at least one MPS nickel layer in addition to said at leastone bright-nickel layer and/or said at least one satin nickel layer,most preferably at least one MPS nickel layer in addition to said atleast one bright-nickel layer and/or said at least one satin nickellayer, and further to said at least one semi-bright nickel layer. In thecontext of the present invention MPS denotes that the MPS nickel layercomprises non-conductive micro-particles, which cause micro-pores in asubsequent chromium layer, preferably in the black chromium layer. Theat least one MPS nickel layer is preferably optionally.

In some cases, a method of the present invention is preferred, whereinthe MPS nickel layer is adjacent to the black chromium layer.

In other cases a method of the present invention is preferred whereinthe black chromium layer is adjacent to the at least one bright-nickellayer and/or the at least one satin nickel layer, which is in many casespreferred, most preferably in combination with the at least onebright-nickel layer.

Preferably, the black chromium layer is part of a layer stack.

In step (b) the substrate, preferably with the at least one nickel layer(preferably as defined above as being preferred) is contacted with theelectroplating bath of the present invention, preferably by dipping.

Preferred is a method of the present invention, wherein the contactingduring step (c) ranges from 1 minute to 30 minutes, preferably from 2minutes to 20 minutes, more preferably from 3 minutes to 15 minutes,even more preferably from 4 minutes to 10 minutes, most preferably from5 minutes to 8 minutes.

Preferred is a method of the present invention, wherein in step (c) theelectroplating bath has a temperature in a range from 25° C. to 60° C.,preferably from 28° C. to 50° C., more preferably from 30° C. to 47° C.This most preferably applies if the electroplating bath compriseschloride ions.

In some cases, a method of the present invention is preferred, whereinin step (c) the electroplating bath of the present invention has atemperature in a range from 35° C. to preferably from 40° C. to 63° C.,more preferably from 45° C. to 61° C., most preferably from 50° C. to59° C. This most preferably applies if the electroplating bath is achloride-free electroplating bath.

In step (c) an electrical current is applied.

Preferred is a method of the present invention, wherein the electricalcurrent is a direct current, preferably in a range from 3 A/dm² to 30A/dm², more preferably from 4 A/dm² to 25 A/dm², even more preferablyfrom 5 A/dm² to 20 A/dm², most preferably from 6 A/dm² to 18 A/dm².

In some cases a method of the present invention is preferred, whereinthe electrical current is a direct current, preferably in a range from 3A/dm² to 20 A/dm², more preferably from 4 A/dm² to 15 A/dm², mostpreferably from 5 A/dm² to 10 A/dm². This most preferably applies if theelectroplating bath is a chloride-free electroplating bath.

Preferred is a method of the present invention, wherein in step (c) atleast one anode is utilized. The at least one anode is selected from thegroup consisting of graphite anodes, precious metal anodes, and mixedmetal oxide anodes (MMOs).

Preferred precious metal anodes comprise platinized titanium anodesand/or platinum anodes.

Preferred mixed metal oxide anodes comprise platinum oxide coatedtitanium anodes and/or iridium oxide coated titanium anodes.

Preferred is a method of the present invention, wherein theelectroplated black chromium layer has a layer thickness ranging from0.05 μm to 1 μm, preferably from 0.1 μm to 0.8 μm, more preferably from0.125 μm to 0.6 μm, most preferably from 0.15 μm to 0.5 μm.

Most important in the context of the method of the present invention isstep (d), the heat-treating. It allows a quick and direct formation ofthe desired black color tone. The temperature applied in step (d) is notthe temperature utilized in step (c) for the electroplating bath. Steps(c) and (d) are distinct steps.

Preferred is a method of the present invention, wherein theheat-treating is at a temperature ranging from 32° C. to 99° C., morepreferably ranging from 45° C. to 92° C., even more preferably rangingfrom 52° C. to 88° C., most preferably ranging from 60° C. to 84° C.

More preferred is a method of the present invention, wherein in step (d)the heat-treating is carried out in water, preferably having atemperature ranging from 32° C. to 99° C., more preferably ranging from45° C. to 92° C., even more preferably ranging from 52° C. to 88° C.,most preferably ranging from 60° C. to 84° C.

As mentioned, the heat-treating is preferably in water. This preferablymeans that this step is carried out in a treatment compartmentcomprising a treatment composition. Preferably, the treatmentcomposition is aqueous, more preferably comprising as solvent onlywater, most preferably essentially consisting of water. Essentiallyconsisting of water means that besides tiny contaminations from previousmethod steps, the main component of the treatment composition is andremains water. Typically, said contaminations are tolerable for thepurpose of this step.

Even more preferred is a method of the present invention, wherein instep (d) the heat-treating is a hot water rinse, most preferably bydipping, even most preferably by dipping into the treatment composition.

Preferred is a method of the present invention, wherein in step (d) theheat-treating is carried out without an electrical current. This meansthat this step is preferably electroless.

The method of the present invention does not exclude further steps,preferably such as additional rinsing, cleaning, pre-treating, and/orpost-treating. Preferably, steps as defined in the examples below applylikewise to the general method described throughout the present text. Apreferred post-treating step comprises a sealing step, preferably withan inorganic and/or organic sealer, and/or a contacting step with ananti-fingerprint composition.

Below, the present invention is illustrated by the followingnon-limiting examples.

EXAMPLES

In the following, Hull Cell electroplating was performed to evaluate theoptical appearance of the black chromium layer depending on the currentdensity distribution.

General Procedure:

As a substrate, copper panels (99 mm×70 mm) were used.

In a first step, the copper panels were cleaned by electrolyticdegreasing with Uniclean® 279 (product of Atotech Deutschland GmbH), 100g/L at room temperature (RT). Afterwards the substrates were rinsed withwater, pickled with 10% H₂SO₄ by volume, and rinsed with water.

In a second step, the cleansed substrates were deposited with a brightnickel layer (10 min, 4 A/dm², UniBrite 2002, product of Atotech) suchthat a nickel-plated substrate was obtained and rinsed with water.

In a third step, the black chromium layer was deposited by utilizing thefollowing electroplating bath:

(A) ca. 20 to 25 g/L Cr³⁺ ions (provided as basic chromium sulfate), (B)ca. 30 g/L Formic acid, (C) ca. 60 g/L Boric acid, (D) Tbl. 1 Potassiumthiocyanate, (E) Tbl. 1 Methionine, ca. 10 g/L Ammonium bromide, ca. 100g/L Ammonium chloride, ca. 100 g/L Potassium chloride, and ca. 0.5 g/LFe SO₄•7 H₂O

The electroplating baths further comprised small amounts (up to 4 g/L)of saccharin and between 5 g/L and 50 g/L of a S-containing diol. Nocobalt ions and no nickel ions were present. Thus, the black chromiumlayer did not comprise cobalt and nickel. However, further experimentsindicate that relatively small amounts of cobalt can be tolerated (notshown).

The pH value was adjusted to 3.2.

Compounds (D) and (E) were utilized in various concentrations andresulting molar ratios as summarized in Table 1 below.

If not stated otherwise, each electroplating bath was tested in a Hullcell having a graphite anode and the nickel-plated substrate wasinstalled as the cathode. An electrical current of 5 A was passedthrough for 3 minutes at temperatures ranging from 35° C. to 45° C. (seeTable 1 for further details).

After plating, the substrates were rinsed with water and dried for afirst color measurement (abbreviated as “CM1” in Table 1). After “CM1”,the substrate was subjected to a hot water rinse for 10 minutes at 70°C. and 80° C., respectively, dried and a second color measurement wascarried out (abbreviated as “CM2” in Table 1).

In a first set of experiments (abbreviated as Examples E1.1 to E1.7 inTable 1), a warm black color tone (b* ranging from ca. +3 to +6) wasimmediately created after the hot water rinse, wherein in a second setof experiments (abbreviated as Examples E2.1 to E2.3 in Table 1) aneutral black color tone (b* about or less than zero) was immediatelycreated after the hot water rinse. Color measurements according to theL*a*b* color space system were carried out with a colorimeter (KonicaMinolta CM-700d; measuring mode: SCI; Observer angle: 10°; Light source:D65) and at a position of ca. 3.5 cm from the left edge of the substrateand 2 cm from the lower edge (representing a typical medium currentdensity (MCD) of approximately 10 A/dm² to 12 A/dm²). ComparativeExamples are abbreviated as “CE”.

Besides above mentioned color measurement, the substrates were alsooptically inspected depending on the locally present current densities(abbreviated as “ASD range” in Table 1). For that, the area ofdefect-free black chromium layer (i.e. having a homogeneous blackchromium layer without haze and burnings) was determined andre-calculated as a corresponding current density range (“ASD range”). Acomparatively broad range of re-calculated current densities isconsidered better because it shows that from low to high currentdensities a defect-free black chromium layer is obtained.

Table 1, electroplating bath compositions and results

(D) (E) (E)/ T*** CM1 CM2 ASD No. [mmol/L] [mmol/L] (D) [° C.] L*; a*;b* L*; a*; b* range Rating E1.1* 101 202 2 70 53; 0.5; 3.8 49; 1.0; 4.93-50 ++ E1.2* 131 306 2.3 70 51; 0.6; 4.6 46; 1.4; 5.6 3-20 ++ E1.3**131 306 2.3 80 50; 1.0; 5.5 44; 1.5; 4.5 5-50 +++ E1.4* 152 202 1.3 7052; 0.6; 4.2 44; 1.5; 4.4 3-25 +++ E1.5* 163 155 1 70 53; 0.5; 3.8 50;0.8; 4.8 3-50 ++ E1.6* 163 206 1.3 70 52; 0.6; 4.4 46; 1.4; 5.2 3-35 +++E1.7* 202 202 1 70 50; 0.7; 4.7 43; 1.6; 3.6 4-25 +++ E2.1** 162 357 2.280 50; 0.8; 5.1 41; 1.6; 0.6 4-50 +++ E2.2** 182 406 2.2 80 48; 1.2; 6.440; 1.0; −1.4 4-20 +++ E2.3** 234 406 1.7 80 48; 1.1; 6.3 41; 0.3; −2.44-15 ++ CE1 49 202 4.1 70 55; 0.4; 3.3 53; 0.6; 4.3 3-50 + *denotes 35°C. electroplating bath temperature **denotes 45° C. electroplating bathtemperature ***denotes rinse water temperature Under “Rating” theover-all performance is evaluated as follows: + denotes that either thecolor measurement (i.e. CM1 and CM2) or the ASD range are meeting therequirements; however, this is not sufficient and therefore not desired;++ denotes that both the color measurement (i.e. CM1 and CM2) and theASD range are meeting the requirements; this is desired; +++ denotesthat both the color measurement (i.e. CM1 and CM2) and the ASD range areexcellently meeting the requirements; this is very much desired CE1shows that a molar ratio of (E)/(D) of 4.1 does not result in asufficiently black/dark color tone (L* = 53). The molar ratio is farexceeding the defined maximum value of 2.65.

Additional Comparative Examples CE2 to CE4 were carried out based on US2020/094526 A1:

-   -   CE2 corresponds to Sample No. 5 in Table 1 (which represents all        samples ranging from 5 to 13 in Table 1), having a (E)/(D) molar        ratio of approximately 2.7 (100 ml/L Trichrome Graphite Makeup        and 30 ml/L Trichrome Graphite Maintenance result in a molar        ratio exceeding 2.65); the color measurement CM1 showed an L*;        a*; b* of 54; 0.5; 3.8 immediately after plating and an ASD        range from 7 to 50. Although the ASD range is relatively broad,        the color is not sufficiently black/dark immediately after        plating.

As shown in US′526, Table 1, a really dark (and neutral black) colortone is obtained only in Example No. 7 with L*; a*; b* of 44; 0.8; 0.4(measured at high current density) with an “Acceleration test”, whichincludes a waiting time under pre-determined conditions for 18 days (seeion US′526). Furthermore, a sufficiently warm black color tone wasobtained only in Example No. 6 (allowed to stand at ambient air for 18days) and Example No. 13 (again with “Acceleration test for 18 days).Own experiments show that a 10 minutes hot water rinse has nosignificant effect on CE2 to obtain a warm black color tone or a neutralblack color tone. Examples 5 to 13 in US'526 therefore have at least thedisadvantage that an idle time (or also called aging time) of 18 or 19days is considered inacceptable in view of industry requirements.However, it is desired to quickly obtain a well-defined black color tone(either a neutral black color tone or a warm black color tone). As shownabove, this can be achieved with the present invention and maintaining acomparatively narrow molar ratio range.

Thus, our own experiments show that a molar ratio of 0.9 to 2.65, basedon (E)/(D) must be maintained to obtain a significant effect caused by ahot water rinse.

-   -   CE3 corresponds to Sample No. 14 in Table 2 in US'526, wherein        the total amount of thiocyanic acid is 15 g/L, i.e. 254 mmol/L,        which results in a (E)/(D) molar ratio of 0.8. This is        significantly lower molar ratio than in Sample No. 5 of US'526.        The color measurement CM1 showed an L*; a*; b* of 47; 1.0; 5.7        immediately after plating at ca. 10 A/dm², which is in good        agreement with what is disclosed in US'526, Table 2, No. 14,        “Initial”. However, irrespective of any hot water rinse, our        experiment also showed that the ASD range is unacceptably narrow        (only ca. 1 ASD) in the Hull Cell setup. In contrast, the        examples of the present invention clearly show that a (E)/(D)        molar ratio range from 0.9 to 2.65 allows not only the desired        color tone but additionally broadens the ASD range. Thus, a        molar ratio below 0.9 very negatively affects the possible        current density range on a substrate and therefore strongly        increases the possibility of plating defects. This can be        surprisingly solved by the present invention by maintaining a        (E)/(D) molar ratio ranging from 0.9 to 2.65.    -   CE4 corresponds to Sample No. 17 in Table 2 of US'526, wherein        the total amount of thiocyanic acid is 40 g/L (i.e. 677 mmol/L),        which results in a (E)/(D) molar ratio of even below 0.4.        Although a deposition is possible, a strong and undesired white        haze is covering major parts of the hull cell substrate        indicating that the range of an acceptable current density range        is even smaller compared to CE3. Thus, CE4 confirms the finding        of CE3 and supports the conclusion that CE4 is not suitable for        electroplating sophisticated or complex substrates which require        a significantly broader current density range.

The examples according to the invention show that either a neutral blackcolor tone or a warm black color tone can be obtained within anacceptable short time. The molar ratio (E)/(D) is chosen in such a wayto obtain an effect from the hot water rinse and that a relative broadcurrent density range is still guaranteed.

Further examples according to the invention were carried out, whereinthe electroplating bath was modified in such a way that no chloride ionswere included (specific data not shown). In these experiments no hullcell experiments were carried out but rather electroplating trials in abeaker with the same hull cell substrates but at a specific currentdensity of 10 A/dm². In these examples a warm black color tone with anL*; a*; b* of 45; 1.3; 3.8 was obtained.

1. An electroplating bath for depositing a black chromium layer, theelectroplating bath comprising: (A) trivalent chromium ions; (B) one ormore than one complexing agent for said trivalent chromium ions; (C)optionally, one or more than one pH buffer compound for saidelectroplating bath; (D) one or more than one compound comprising atleast one —SCN moiety, salts, esters, and/or isoforms thereof, in atotal amount ranging from 100 mmol/L to 750 mmol/L, based on the totalvolume of the electroplating bath; and (E) one or more than one organiccompound, including sulfoxides thereof, comprising at least one —SHmoiety and/or at least one —S—(CH₂)_(k)—CH₃ moiety, wherein k is aninteger ranging from 0 to 4, characterized in that (E) and (D) arepresent in a molar ratio ranging from 0.9 to 2.65, based on (E):(D). 2.The electroplating bath according to claim 1, wherein the electroplatingbath further comprises sulfate ions.
 3. The electroplating bathaccording to claim 1 further comprising halogen anions.
 4. Theelectroplating bath according to claim 1 further comprising Fe(II) ions.5. The electroplating bath according to claim 1, wherein (C) comprisesboric acid.
 6. The electroplating bath according to claim 1, whereinsaid bath comprises (D) in a total amount ranging from 100 mmol/L to 600mmol/L, based on the total volume of the electroplating bath.
 7. Theelectroplating bath according to claim 1, wherein said bath comprises(E) in a total amount ranging from 1 mmol/L to 950 mmol/L, based on thetotal volume of the electroplating bath.
 8. The electroplating bathaccording to claim 1, wherein (E) comprises at least a compound offormula (I), salts, and/or sulfoxides thereofR¹—S—(CH₂)_(n)—CH(NH₂)—R²  (I), wherein R¹ is a branched or unbranchedC1 to C4 alkyl, R² is selected from the group consisting of COOH, saltsthereof, and (CH₂)_(m)—OH, n is an integer ranging from 1 to 4, and m isan integer ranging from 1 to
 4. 9. The electroplating bath according toclaim 8, wherein R¹ is methyl, ethyl, n-propyl, or iso-propyl.
 10. Theelectroplating bath according to claim 8, wherein R² is COOH and/orsalts thereof.
 11. The electroplating bath according to claim 8, whereinn is 1 or
 2. 12. The electroplating bath according to claim 1, wherein(E) comprises at least methionine.
 13. The electroplating bath accordingto claim 1, wherein the molar ratio of (E):(D) is ranging from to 2.6.14. A method for electroplating a black chromium layer on a substrate,the method comprising the steps (a) providing the substrate, (b)contacting the substrate with an electroplating bath according to claim1, (c) applying an electrical current such that the black chromium layeris electroplated onto the substrate, (d) heat-treating the substrateobtained from step (c) at a temperature ranging from 30° C. to 100° C.15. The method of claim 14, wherein in step (d) the heat-treating iscarried out in water.